Here's a secret known to all car detailers and very few used-car buyers. The best way to check out any car is to wash it.

Buyer's tip: A ruse for the used

In 30 minutes you'll know the car almost as intimately as the owner. You'll know every scrape and every misaligned panel.

You'll know if the doors have sunk, if the carpets have been replaced and if the spare wheel is rusty and wearing a bald tyre.

And, with soft-tops, you'll know about leaks. They used to be the way of life with soft-tops but lately weather sealing has become excellent.

If you can't wash the car at least simulate heavy rain with a garden hose (when water restrictions aren't on ...), both coming down vertically and being blown up the windscreen and along the side windows. Now climb inside and go around all the interior surfaces adjacent to the roof and directly underneath where it joins the body.

If water is intruding it's most likely along the tops of the windows where older soft-tops sag. You can live with a few drops here and there, but not a stream of water in your lap.

First, however, operate the top and note where all the surfaces fold or catch the frame during the process. Check in these places for fraying and wear. Put your eye against them and see if there are pinpricks of sunlight coming through, or if they're thinner than the rest of the material.

Now stick your nose in the roof's storage bin and sniff for mildew or anything that suggests the roof has been stored while wet.

Then have a look at the rear window. If it's not glass it will mark easily and wear, and it costs a lot to replace. That's why you see so many MX-5s without one -- it's so scored and cloudy that it obscures the rear view.

Plenty of soft-tops now have electronic roof actions and you need to stand back and watch it while someone else presses the buttons. It should be smooth and solid.

Some movements have a bit of jerkiness but too much is a danger sign, as is one side moving slightly faster than the other.

If the seller gives it any assistance at all (apart from clinching and unclinching the clasps at the top of the windscreen) you'll shortly have some spending to do. Ditto if there are nasty noises.

Cars with folding steel roofs have less dramas but the leak, fold and mildew tests are still applicable, as well as checking for fraying or wear along the folds you're looking for rust.

It's a funny thing with soft-tops -- some people never put the roof down. If it has been in place for four or five years the joints may be locked up -- you'll need a lot of lubricant (like WD40) to get them moving again.

The final point to look for is modification. Many owners modify their convertibles, some of them not well, so beware of alterations to the factory specification especially with the engine and suspension and, to a lesser extent, with the wheels and body.